Visible Light Communication is a branch of communications research dealing with communicating data via visible light. It has gained popularity in recent years due to the increasing prevalence of light emitting diodes (LEDs), which have special properties that make them outstanding sources (and receivers) for data via visible light. LED has the special characteristic of being able to turn the light on and off very fast. Data can then be transmitted by turning LEDs on and off at ultra high speed.
The basic idea is that this light resource, which is commonly used to merely transmit visible light to people can simultaneously be used to transmit data. As LEDs grow even more prevalent, it becomes possible for ubiquitous amounts of light sources to be used as data transmission resources. Light fixtures, traffic lights, brake lights, active billboards, etc. can all be used to transmit information rather than just light.
In one example, gas can be saved on a car or truck by turning the engine off while waiting at red lights. Currently, however, it is impractical to do so, as the delay in starting the car or truck back up when the light turns green would increase traffic. If the traffic lights were equipped with LEDs, however, as is already becoming prevalent for energy savings purposes, the traffic lights could be configured to send a data signal to a car informing it of an impending transition between red and green. The car could then be configured to start up and be ready for forward movement as soon as the light turns green.
Such visible light communication also has advantages over other types of wireless transmission. Radio Frequency (RF) data transmission, for example, is essentially omnidirectional, which would limit its ability to focus the data to particular locations. In the traffic light example, using RF would be impractical because cars traveling in other directions (or even at traffic lights) would all receive the same signal. The line-of-sight aspect of visible light communication prevents this type of “data transmission leakage.” Another advantage is that RF communications have power constraints, either set by the Federal Communications Commission (FCC) or just inherent in the fact that high power may be needed to transmit at certain frequencies. Visible light communication suffers from no such power constraints. Another advantage is that LEDs are already becoming prevalent as light sources merely for their power savings features, and these LEDs can be used to transmit the visible light communication without needing to replace large amounts of hardware. Furthermore, RF transmissions are prohibited in certain locations, such as on or near operating aircraft, and in hospitals, due to potential interference with electronic equipment. Visible light communications are not subject to these prohibitions.
Visible light communication also has advantages over Infrared (IR) communication. Both visible light and IR communications have the similar bandwidth (a few 100 THz). However, Infrared Data Association (IrDA) communication has an eye safety problem in conjunction with the possible dangerous high energy density due to its invisibility. Therefore, higher data rate transmissions cannot be accomplished through IR communication. As compared to IR communication, the visible light communication is more suitable to human eyes in terms of “Visibility”, and is capable of transmitting data at a much higher rate.
Another advantage of Visible Light Communication over IR is that, since the communication beam can be seen, the location of where the beam is pointing can be observed, which aids in determining if the intended recipient can receive the beam as well as aid in preventing improper recipients (hackers, eavesdroppers, etc.) from receiving the beam.
One unusual (and not well known) fact about LEDs is that they can be used not only to transmit visible light communication but also to receive them. This widens even further the potential applicability of visible light communication. In the traffic light example described above, a car sitting behind a truck at a traffic signal may not have direct line of sight to the traffic light, and therefore direct data transmission between the traffic light and the car may be blocked. However, the various LEDs in the “system” can essentially be daisy-chained together. The trucks headlights can act as a receiver to receive the data from the traffic light, and then the trucks taillights can retransmit this data, which can then, in turn, be received by the car's headlight.
IEEE 802.15.7 is a standards body working group that is putting together a common standard for visible light communication. Current methods for visible light communication encode the data using a scheme that sets the output duty cycle of the encoded data to a fixed value of 50%. At this fixed amount, brightness would be at a constant level. Changes in brightness are accomplished by inserting frames into the output stream that do not represent data at all, but instead contain patterns that gave a duty cycle other than 50%. These “blank” frames do not contain data at all, but are used exclusively to control the brightness of the light source. This allows the overall brightness of the light source to be controlled.
The problem with this approach, however, is that the blank frames use up a portion of the transmission time that could otherwise be used to transmit data. This reduces the potential throughput of the system, and in fact the further away the system needs to get from 50% the worse the throughput gets (as more blank frames are needed to change the perception of brightness). Furthermore, this solution adds complexity to the transmitter, in that in order to precisely control the overall brightness, the duty cycle of the patterns in the blank frames will have to compensate for the 50% duty cycle of the data carrying frames. In order to perform this compensation, the transmitter will have to know much data is being transmitted.